Hearing, both normal and pathological, involves the processes of discriminating an auditory stimulus and attending to it. Ascending auditory pathways analyze the physical characteristics of a stimulus, but descending pathways control the access of the information carried in the ascending pathways to higher auditory centers. Most studies to date of these descending pathways have focused on efferent axons reaching the cochlea. This project is designed to investigate efferent systems at the level of their projection to the first central synapse in the auditory pathway, i.e., the cochlear nuclei. Morphological evidence indicates that the cochlear granule cell area is the target of a number of efferent projection systems, and behavioral evidence is accumulating which strongly suggests that this area is involved in alerting and attention to auditory stimuli. The long term goal of this program of investigation is to clarify the role of the cochlear granule cell system in directed attention to auditory stimuli. The immediate goal is to determine the type of afferent information the granule cell system receives, by identification of the types of input to the area and study of its synaptic organization. This will be accomplished by studies involving: (1) Golgi impregnation to identify classes of neurons and to determine the extent of their dendritic fields; (2) retrograde transport studies to identify all sources of input to the granule cell area; (3) immunohistochemistry, done partly in conjunction with retrograde transport, to characterize the cholinergic, GABAergic, and noradrenergic input systems; and (4) electron microscopy to characterize types of synaptic terminals in the granule cell area and to describe two types of direct cell-cell junctions between neurons in this area.